This invention relates to an air conditioning system for use in a vehicle, which is adapted to reduce the waste of energy of the compressor.
In an air conditioning system adapted for use in an automotive vehicle, a compressor is connected to an engine installed in the vehicle, through an electromagnetic clutch, to be driven by the engine to compress refrigerant and feed the compressed refrigerant to a condenser for liquefying same. Liquefied refrigerant from the condenser is reduced in pressure by an expansion valve and delivered to an evaporator arranged in an air passage whereby it vaporizes to cool air flowing in the evaporator. A blower is arranged in the air passage upstream of the evaporator and feeds circulated air from the passenger compartment of the vehicle or fresh air to the evaporator, and cooled air from the evaporator is discharged into the passenger compartment to cool the interior thereof. On the other hand, the vaporized refrigerant from the evaporator is again delivered to the compressor for recirculation throughout the above-mentioned component parts. The above cycle of refrigeration is repeated to thereby cool the passenger compartment.
In the air conditioning system arranged as above, the temperature of cooling fins of the evaporator is detected and compared with a set value, and depending upon the result of comparison is the compressor actuated or stopped through engagement or disengagement of the electromagnetic clutch, to thereby control the temperature of air being discharged into the passenger compartment to a desired value.
According to such conventional arrangement wherein the compressor is connected to or disconnected from the engine by operating the electromagnetic clutch, each time the clutch is engaged or disengaged, a shock is produced by the clutch and applied to the vehicle body, creating an unpleasant feeling to the driver and the passenger, and spoiling the driveability of the vehicle.
Such disadvantage might be overcome by employing, in place of the conventional compressor using the electromagnetic clutch, a compressor which is adapted to automatically maintain a constant suction pressure (hereinafter called "the suction pressure-automatic control type"), as proposed in U.S. Pat. No. 3,861,829 issued Jan. 21, 1975. The use of the suction pressure-automatic control type compressor will avoid the occurrence of a shock upon engagement or disengagement of the electromagnetic clutch of the aforementioned conventional compressor, improving the driveability of the vehicle, etc. Besides, the use of the suction pressure-automatic control type compressor will somewhat contribute to curtailment of the energy, since the compressor is operated with an appropriate capacity required to maintain the suction pressure at a predetermined fixed value.
However, there are difficulties in actually replacing the electromagnetic clutch type compressor with the suction pressure-automatic control type compressor. That is, in the latter compressor, the predetermined fixed value to which the suction pressure is to be controlled is set at a value equal to or in the vicinity of a limit value below which freeze-up of the evaporator can take place, so as to achieve sufficient cooling. Further, the predetermined fixed value of suction pressure cannot be optionally adjusted once it has been set. Therefore, the discharge air temperature can drop below a desired value set by the driver or the passenger, resulting in excessive cooling. In such an event, it will be necessary to provide a heater core for heating air once cooled by the evaporator (reheat) to increase the discharge air temperature to a desired temperature. This impedes attainment of expected energy saving with the suction pressure-automatic control type compressor, to an extent excelling the electromagnetic clutch type compressor.
In order to adapt a compressor of the suction pressure-automatic control type to the conventional air conditioning arrangement while at the same time preventing excessive cooling, there has been proposed a refrigeration control system by U.S. Pat. No. 4,112,703 issued Sept. 12, 1978, which employs a compressor of the suction pressure-automatic control type and is adapted to control the refrigerating capacity of the evaporator so as to avoid excessive cooling, thereby dispensing with a heater core for reheating the cooled air from the evaporator. More specifically, in order to control the refrigerating capacity of the evaporator, the proposed control system is provided with an electrically operated solenoid valve as the expansion valve and detects the superheat degree of refrigerant gas at the outlet of the evaporator. In response to the detected superheat degree, the duty cycle of the coil of the solenoid valve is controlled to adjust the valve opening thereof to thereby control the flow rate of refrigerant to be supplied to the evaporator for adjustment of the superheat degree of refrigerant gas at the outlet of the evaporator. Thus, eventually the refrigerating capacity of the evaporator is controlled to maintain the compartment temperature at a desired set value.
However, according to the proposed control system, the control section is complicated in construction, resulting in increased manufacturing costs. The on-off type solenoid valve can undergo hunting, leading to unstable control operation. Further, since the flow rate of refrigerant supplied to the evaporator is varied, the superheat degree is apt to increase, and accordingly the temperature of discharge refrigerant of the compressor can rise, which can cause seizure of component parts of the compressor, etc.